This invention relates to a driving circuit of a solenoid actuated device wherein a plunger is moved in the axial direction of a cylinder by passing current through a solenoid coil wound about the cylinder.
One example of such solenoid actuated device is disclosed in Japanese laid open patent specification No. 31427/1977 dated Mar. 9, 1977. The device disclosed therein is used to control the opening of an oil pressure control valve in accordance with the running speed of a motor car. A driving circuit is provided to pass current through a solenoid coil corresponding to the running speed.
In order to move a plunger which is operatively connected to an oil pressure control valve it is necessary to pass a current of about 0.6 ampere through the solenoid coil from a source of 16 V, for example, and the solenoid coil is connected to the output of a transistor. Considering the loss of the transistor, the source is required to have a capacity sufficient to supply the necessary power to the solenoid coil and the power loss of the transistor. Such output transistor is necessary to be provided with heat radiating fins so that it size is not only bulky but also requires a large space.